Axial heteroatom (P, S and Cl)-decorated Fe single-atom catalyst for the oxygen reduction reaction: a DFT study

An FeN4 single-atom catalyst (SAC) embedded in a graphene matrix is considered an oxygen reduction reaction (ORR) catalyst for its good activity and durability, and decoration on the Fe active site can further modulate the performance of the FeN4 SAC. In this work, the axial heteroatom (L = P, S and Cl)-decorated FeN4 SAC (FeN4L) and pure FeN4 were comparatively studied using density functional theory (DFT) calculations. It was found that the rate-determining step (RDS) in the ORR on pure FeN4 is the reduction of OH to H2O in the last step with an overpotential of 0.58 V. However, the RDS of the ORR for the axial heteroatom-decorated FeN4L is the reduction of O2 to OOH in the first step. The axial P and S heteroatom-decorated FeN4P and FeN4S exhibit lower activity than pure FeN4 since the overpotentials of the ORR on FeN4P and FeN4S are 1.02 V and 1.09 V, respectively. Meanwhile, FeN4Cl exhibits the best activity towards the ORR since it possesses the lowest overpotential (0.51 V). The main reason is that the axial heteroatom decoration alleviates the adsorption of all the species in the whole ORR, thus modulating the free energy in every elementary reaction step. A volcano relationship between the d band center and the ORR activity can be determined among the axial heteroatom-decorated FeN4L SACs. The d band center of the Fe atom in various FeN4L SACs follows the order of FeN4 > FeN4Cl > FeN4S > FeN4P, whereas the overpotential of the ORR on various catalysts follows the order of FeN4Cl > FeN4 > FeN4S ≈ FeN4P. ΔG(*OH) is a simple descriptor for the prediction of the ORR activity of various axial heteroatom-decorated FeN4L, although the RDS in the ORR is either the first step or the last step. This paper provides a guide to the design and selection of the ORR over SACs with different axial heteroatom decorations, contributing to the rational design of more powerful ORR electrocatalysts and achieving advances in electrochemical conversion and storage devices.


Introduction
A typical example of a clean energy conversion technology is the proton exchange membrane fuel cell (PEMFC).To ensure energy conversion, the sluggish oxygen reduction reaction (ORR) at the cathode needs a powerful electrocatalyst. 1 Currently, the most efficient catalyst for the ORR is platinum group metals (PGMs).However, their major drawbacks (such as huge overpotential, scarcity, and high cost) limit their commercialization.One effective way to address this issue is to explore alternatives to non-precious metal catalysts.Carbon materials doped with metals and heteroatoms have drawn much interest from researchers as promising and exceptional electrocatalysts to efficiently reduce the overpotential of the ORR.3][4] As opposed to PGMs and carbon-based metal-free catalysts, singleatom catalysts (SACs) have emerged as a frontier in the eld of ORR electrocatalysts owing to their large specic surface area, maximum atomic efficiency, high selectivity, and long-term operational endurance.DFT calculations have been widely and signicantly successful in research studies on the ORR mechanism 5 since it can greatly save experimental costs and accurately predict the components with high activity.Graphene with defects is one of the numerous substrates that can make an excellent SAC substrate.8][9] Both experimental studies and DFT calculations have shown that the SACs based on non-precious TM (TM = Fe, Co, Mn, Ni, Zn, etc. 2,[10][11][12] ) on nitrogen-doped carbon support can signicantly enhance the ORR electrocatalytic activity.Among the aforementioned SACs, the single-atom Fe-N-C catalyst exhibits considerable activity and remarkable selectivity for the ORR, 9,13,14 which is even comparable to the commercial platinum catalysts.Furthermore, our previous experiments have proved that the activity and stability of FeN 4 is as good as the platinum catalyst for ORR. 15,16The FeN 4 catalyst has been reported as a possible replacement for Pt-based catalysts because of its good activity and durability, and the decoration on the Fe active site can further improve the performance of SACs through modulating the electronic and geometric structure. 17,18Thus, the rational design of carbon-based ORR catalysts with low cost, as well as high activity, selectivity, and durability, is of great signicance.Lin et al. 19 synthesized a new SAC, and each Fe atom is coordinated with ve N atoms rather than four N, resulting in a 3D Fe-N 5 coordination site.By using DFT calculations, they discovered that the axially coordinated pyridine could positively regulate the binding strength of oxygen on the 2D Fe-N 4 site to a higher level.Other scholars came to similar results that were well-researched. 20,21In addition, SACs exhibited good activity for electrochemical N 2 reduction, 22 CO 2 reduction, [23][24][25] lithiumsulfur batteries, 26 NO electroreduction 27,28 and CO detection. 29hus, the study on axial heteroatom-decorated Fe SAC for ORR is important, and this paper can shed light on the rational design of SAC catalysts with good performance.
In this work, DFT calculations were performed to investigate the effect of different axial heteroatom decorations on the activity of FeN 4 L towards ORR.The binding energy and cohesion energy, which describe the stability of the heteroatomdecorated FeN 4 L, were rst studied.Then, the overpotentials of ORR catalyzed by pure FeN 4 and various FeN 4 L were evaluated.1][32] An accurate quantitative description of the ORR activity trends was elucidated with the help of the adsorption free energy of the reaction intermediate, d-band center, partial density of states, differential charge density and Bader charge of the heteroatomdecorated Fe atom.

Computational methods
The spin-polarized DFT calculations were performed by using the Vienna Ab initio Simulation Package (VASP). 33,34The Kohn-Sham wave functions were expanded in a plane wave basis set with a cutoff energy of 500 eV.The projector-augmented wave (PAW) method and PBE potential for the exchange-correlation function were used. 35The 3 × 3 × 1 Monkhorst-Pack k-point mesh was used to sample the Brillouin zone. 36All atoms were allowed to relax until the forces fell below 0.02 eV Å −1 .A vacuum region of 15 Å was created to ensure negligible interaction between mirror images.
The ORR occurs via the following steps: O 2 (g) + * + H + + e − / *OOH (1) *O + H + + e − / *OH (5) *OH + H + + e − / * + H 2 O (7) where The Gibbs free energy (DG) diagram of the ORR was calculated using the computational hydrogen electrode (CHE) model proposed by Nørskov et al., 37 where the free energy of (H + + e − ) under standard conditions is equal to the value of 1/2H 2 .The free energy change (DG) of the primitive step of the ORR is calculated according to the following equation.
where DE is the total energy difference between the reactants and products of the reactions, DZPE is the zero-point energy correction, DS is the vibrational entropy change and T is temperature with the value of 298.15 K. D Ð C P dT is the difference in enthalpic correction, DG U = −eU, where e is the elementary charge, U is the electrode potential, and DG pH is the correction of the H + free energy.
The overpotential for ORR (h ORR ) is calculated according to the following equation: where 1.23 V is dened as the equilibrium potential of the overall 4-electron ORR at the standard state, and DG max represents the maximum DG associated with the proton-electrontransfer steps.
The binding energies (E Binding ) of the axial heteroatom with FeN 4 and the adsorption energy (E ads ) of the intermediates in ORR have been calculated according to formulas (11) and ( 12): where E FeN 4 X , E FeN 4 and E X are the total electronic energies (non-ZPE corrected) of the system, FeN 4 and heteroatoms, respectively.E system , E C and E species denote the total electronic energy (non-ZPE corrected) of the adsorption system, the individual surface and adsorbate, respectively.As shown in Table 1, the binding energy between the heteroatom and Fe are all negative with values of −1.79 eV, −3.30 eV, and −2.97 eV, respectively, indicating that these structures can be stabilized.Thus, the subsequent adsorption of the ORR intermediates can be carried out on these structures.

Result and discussion
The At the ORR equilibrium potential, all four DG values are negative, indicating that each reaction of the ORR is thermodynamically favoured to occur.For a perfect ORR catalyst, DG should be −1.23 eV for each elementary reaction step involving electron transfer, causing zero overpotential (Fig. 2a).The rate-    S3 †).As a transition metal element, the central Fe atom can transfer partial electrons throughout the ORR process, resulting in stronger binding to diverse intermediates.Thus, the obtained conclusion is consistent with the above calculation results, i.e., at the central iron atom, some electron transfer occurs, followed by hydroxide ion desorption.On the contrary, aer the central iron atom has been axially decorated with heteroatoms, a portion of the electrons are transferred to the heteroatoms.The electrons provided by the iron atom to the oxygen reduction intermediates are reduced, and the binding becomes weaker, making desorption of the hydroxide ion easier on FeN 4 X.Conversely, the adsorption of O 2 to generate OOH is the most difficult step in the entire ORR process.
Table 3 demonstrates that the adsorption of various intermediates of the ORR on FeN    38 The DG ads of *OOH, *O, and *OH can be calculated according to the following equations: Table S1 † lists the calculated DG(*OH), DG(*O), and DG(*OOH) on various catalysts.DG(*OH) is the variable used to describe the ORR overpotential because RDS is related to either DG(*OH) or DG(*OOH).As DG(*OH) becomes more negative, the strength of the bond between *OH and the catalyst increases.
The adsorption free energy is not equivalent to the adsorption energy, but reects the trend of the binding strength of the same substance on different substrates.An appropriate catalyst should have a reasonable DG ads to make sure that the    The electronic properties of a substance have a signicant impact on its catalytic activity.The study of the electronic properties of catalysts can provide a greater understanding of

Conclusions
We performed a detailed calculation and analysis of the structural stability, electronic property, and catalytic activity of both FeN 4 and axial heteroatom-decorated FeN 4 L (L = P, S and Cl) using DFT calculations.Our results show that all of the axial heteroatom-decorated FeN 4 L SACs have good structural stability.The strong adsorption of the reaction intermediates on FeN 4 resulted in suboptimal catalytic activity for ORR, and the RDS of ORR on FeN 4 is the reduction of OH to H 2 O with an overpotential of 0.58 eV.For the axial heteroatom-decorated FeN 4 L, all of the heteroatoms can bind tightly with the central Fe atom in FeN 4 .Meanwhile, the RDS for FeN 4 P, FeN 4 S and FeN 4 Cl is the hydrogenation of O 2 to OOH with the overpotentials of 1.02 V, 1.09 V and 0.51 V, respectively.FeN 4 Cl is nearly at the top of the volcano diagram, and the overpotential of ORR is closest to Pt(111).The axial Cl heteroatom decoration improves the ORR catalytic activity compared with pure FeN 4 , while the axial P and S heteroatoms have a negative effect on the ORR.The evaluation of ORR activity via DG(*OH) is applicable for the axial heteroatom-decorated FeN 4 L, although the RDS in the ORR is either the rst step or the last step.

A 6 ×
6 graphene supercell containing 72 carbon atoms was used as the basis for the construction of the axial heteroatomdecorated FeN 4 SAC.A double carbon atom vacancy was constructed at the center of graphene.Then, an Fe atom was embedded in the center of the double vacancy, and the four closest C atoms surrounding the vacancy were replaced by four N atoms, resulting in a structural model of the FeN 4 catalyst.Fig. 1 shows the top and side views of FeN 4 , FeN 4 P, FeN 4 S, and FeN 4 Cl.The metal and the four coordinated N atoms almost remain in place in the pure FeN 4 , while a slight deviation from the original plane can be determined aer the axial heteroatom decoration.The Bader charge in Fig. S1 † shows that the central Fe atom in FeN 4 , FeN 4 P, FeN 4 S, and FeN 4 Cl loses electrons (−1.03e, −0.86e, −1.06e, and −1.15e; the negative value represents the deciency of electrons), and electrons are transferred to the surrounding coordinated nonmetal atoms, where the P, S, and Cl atoms get 0.04e, 0.44e, and 0.54e, respectively.
optimized structures of the *OOH, *O, and *OH intermediates adsorbed on FeN 4 , FeN 4 P, FeN 4 S, and FeN 4 Cl are depicted in Fig. S2.† The Fe atom is located at the center of the double vacant graphene in the FeN 4 , FeN 4 P, FeN 4 S, and FeN 4 Cl systems.The bond lengths between the central Fe and 4 coordinated N atoms are almost similar, and the deviation of the central Fe atom from the graphene plane became smaller compared with the pure FeN 4 .Table 2 lists the bond lengths between the central Fe and various O atoms from the *OOH, *O, *OH intermediates and the inter-bond lengths of various ORR intermediates as well.For the adsorbed *OOH intermediates on various axial heteroatom FeN 4 L, the bond lengths of Fe-O follow the order of FeN 4 S > FeN 4 P > FeN 4 Cl > FeN 4 , and the values are 2.02 Å, 1.95 Å, 1.86 Å, and 1.80 Å, respectively.This is consistent with the fact that the relatively large binding energy usually corresponds to a short distance.For the *O intermediates, a similar phenomenon can also be observed.The bond lengths of Fe-O follow the order of FeN 4 S > FeN 4 P = FeN 4 Cl > FeN 4 , and the values are 1.69 Å, 1.68 Å, 1.68 Å, and 1.65 Å, respectively.Similarly, for the *OH intermediate, the bond lengths of Fe-O follow the order of FeN 4 S > FeN 4 P > FeN 4 Cl > FeN 4 , and the values are 1.94 Å, 1.92 Å, 1.87 Å, and 1.83 Å, respectively.Thus, it can be concluded that the axial heteroatom decoration of FeN 4 L leads to a decreased binding between various ORR intermediates and axial heteroatom-decorated Fe SACs.The free energy changes (DG) on the different catalysts are shown in Fig. 2. The DG for each elementary step at the active sites of various FeN 4 L are based on the following four reactions: (1) O 2 adsorbs at the active site and hydrogenates to form *OOH; (2) *OOH continues to hydrogenate to form *O; (3) *O continues to combine with H + and forms *OH; (4) H 2 O is produced through hydrogenation of *OH and eventually desorbs.
step (RDS) of ORR on FeN 4 differs from that on the axial heteroatom-decorated FeN 4 L, as illustrated in Fig.2.The RDS is dened by eqn(10).For FeN 4 , the reaction is determined by the last step of ORR, i.e., the formation of H 2 O from *OH (DG(4)).However, for the axial heteroatom-decorated FeN 4 P, FeN 4 S, and FeN 4 Cl, the reaction is controlled by the rst proton-electron transfer step (DG(1)), i.e., the protonation of oxygen to *OOH.At 1.23 V, the two steps of hydrogenation of *O and the formation of H 2 O from *OH are free energy increasing processes at the FeN 4 active site, with the largest free energy increase for *OH.This suggests that the *OH removal step is the RDS with an overpotential of 0.58 V. Whereas the energy of the rst step for FeN 4 P, FeN 4 S, and FeN 4 Cl is 1.02 eV, 1.09 eV, and 0.51 eV, respectively.The FeN 4 Cl has the lowest overpotential, indicating that the axial Cl decoration contributed the most to the ORR activity of FeN 4 L compared to the pure FeN 4 structure, the P and S heteroatom-decorated FeN 4 L. Fig. 3 delves deeper into the ORR catalytic cycle and the optimized conformation of the ORR intermediates on FeN 4 and FeN 4 Cl to reveal the origin of their high ORR activity (FeN 4 P and FeN 4 S in Fig.

4 ,
FeN 4 P, FeN 4 S, and FeN 4 Cl is an exothermic process.The adsorption of OOH was strongest on the FeN 4 active site with an adsorption energy of −1.68 eV, moderate adsorption energy of −1.12 eV on FeN 4 Cl, and the smallest adsorption energy on the FeN 4 S surface with a value of −0.53 eV.Meanwhile, it is −0.60 eV on FeN 4 P. Correspondingly, O also exhibited the strongest adsorption on the FeN 4 active site with −4.36 eV, followed by FeN 4 Cl at −3.52 eV.The adsorption energies on the FeN 4 P and FeN 4 S surfaces are also relatively strong at −2.46 eV and −2.41 eV, respectively.Similarly, the exothermic processes that occur during the OH intermediate adsorption on the surfaces of FeN 4 , FeN 4 P, FeN 4 S, and FeN 4 Cl are all thermodynamically advantageous.*OH has the highest adsorption energy (−2.73 eV) on FeN 4 , the lowest (−1.68 eV) on FeN 4 P, and −1.74 eV and −2.23 eV on FeN 4 S and FeN 4 Cl, respectively.In comparison, OOH, O, and OH on FeN 4 have the strongest adsorption.Meanwhile, FeN 4 P, FeN 4 S, and FeN 4 Cl, which were decorated by axial heteroatoms, decreased the adsorption of the oxygen-containing ORR intermediates to varying degrees.Furthermore, the adsorption energy of the *O intermediate was higher than both *OOH and *OH on the same catalyst.This indicated that the dissociation of OOH in the rst step is relatively simple, and that the hydrogenation reaction of *O occurring in the latter step is also favored by the strong adsorption of the intermediate O.32

Fig. 2
Fig. 2 Free energy diagrams for the ORR on FeN 4 , FeN 4 P, FeN 4 S, FeN 4 Cl, and the ideal catalyst at (a) U = 0 V and (b) U = 1.23 V.

Fig. 3
Fig. 3 Schematic diagram of the cycle of the ORR.The middle diagram shows DG of the ORR at different potentials on (a) FeN 4 and (b) FeN 4 Cl.For U < 0.65 V and U < 0.72 V, all steps on FeN 4 and FeN 4 Cl are thermodynamically accessible.

Fig. 4
Fig. 4 (a) Volcano plot between the theoretical overpotential (h ORR ) and the adsorption free energy of *OH for the ORR on various catalysts.The red dashed line represents the ORR overpotential on Pt(111).(b) The relationship between the adsorption free energy of *OOH and *OH.The ideal intersection point is marked with an orange cross.

Fig. 7
Fig. 7 (a) Projected density of states of the Fe atom on FeN 4 , FeN 4 P, FeN 4 S and FeN 4 Cl, and their corresponding d-band centers.(b) Relationship between the d-band center of the Fe atom in FeN 4 , FeN 4 P, FeN 4 S and FeN 4 Cl and DG(*OH).

Table 1
Binding energy (E Binding ) between the axial heteroatom and Fe in different FeN 4 L © 2024 The Author(s).Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 16379-16388 | 16381

Table 2
Bond length (Å) between the ORR intermediates OOH, O, and OH and the central active Fe site

Table 3
The adsorption energy (E ads ) of OOH, O, and OH on FeN 4 , FeN 4 P, FeN 4 S, and FeN 4 Cl